There is an increase each year in the number of hip and knee total joint replacement surgeries, respectively total hip arthroplasties (THA) and total knee arthroplasties (TKA). Recently the number of surgeries exceeded 600,000 operations a year in the United States.
The cost of an initial total hip replacement remains high. Revision surgeries to replace a failed hip prosthesis are typically more difficult and consequently more expensive. The annual cost for a 3% revision rate can be estimated to reach approximately $1 billion in the U.S. There is a strong need to minimize any conditions that lead to failure of the initial surgery.
In failed total hip arthroplasties with cemented stems, it is estimated that 20% of revision surgeries result from loss of fixation at the interface between the bone cement and the metallic femoral stem component.
There is clear indication that indicates that excessive voids caused by bubbles at the interface of the bone cement and the stem component (“interfacial porosity”) leads to failure of the joint between the stem component and the bone cement. This interfacial porosity reduces the area over which loads are transferred from the implant to the cement mantle, resulting in local stresses that exceed the yield strength of the interface. It is known that the degree of interfacial porosity is primarily controlled by the rheology of the curing cement and the interaction of the curing cement with the stem component during insertion.
In hip and knee total arthroplasties, surgeons often use bone cement to fix the femoral stem component or tibial component of the prosthesis in the respective bone. The bone cement is often in the form of a two-part polymethyl methacrylate grout. The surgeon mixes pre-polymerized polymethyl methacrylate (PMMA) beads with methyl methacrylate monomer in the presence of chemicals that initiate a free radical polymerization reaction. When the cement is partially polymerized, or “cured”, so that the liquid cement is viscous enough to be retained in a reamed cavity in the tibia or femur it is injected under pressure into the cavity. After polymerization has proceeded for an additional determined period, the surgeon inserts the stem component of the prosthetic joint into the partially cured cement. The cement then fully cures, fixing the stem component in place.
Clinical studies of failed hip arthroplasties have shown that a large number of failures occur at the cement-stem interface. The cyclical loading pattern imposed on the interface between the cement and metallic stem makes them susceptible to fatigue crack growth. Failure analysis on bone cement specimens subjected to fatigue testing shows that crack formation often forms at pores, or gas pockets in the cement. Centrifugation and vacuum mixing are now commonly used to reduce pores in the bulk of the cement, but these procedures do not reduce the interfacial porosity to levels below or equal to the bulk porosity. An intact cement-stem interface will assure an even distribution of the applied load, and will consequently decrease stress concentration and reduce the likelihood of cement fracture.
Porosity at the interface between the cement and the stem component of the prosthesis can be a major cause of the failure of cemented prostheses. A study of the cement mantles from retrieved hip prostheses showed that the porosity at the interface between the cement and the stem component of the prosthesis was much higher than the porosity in the bulk cement. Controlled experiments with differing stem materials showed that the interfacial porosity of the cement did not depend on which metal was used to form the stem component of the prosthesis, but may be more related to the rheology, or flow behavior, of the bone cement.
A cement with a lower viscosity will fully contact the surface of the prosthesis and fill the areas left by displaced air. However, interfacial porosity can be more concentrated at the distal and proximal portions of the prosthesis, which is where failure usually occurs.
Others have examined cement rheology by evaluating stem components that were inserted into the cement at a stage when the cement was more fully polymerized. Thus, the viscosity of the cement was higher and the cement had more elastic behavior and tended to form more interfacial pores. Conversely, it was found that model stem components that were inserted into cement at a lower viscosity stage had a lower number of interfacial pores. The results of this study indicate the benefits of injecting the cement into the bone cavity, and later inserting the orthopedic implant into the cement-filled cavity, when the cement still has a lower viscosity.
However, hip and knee surgeries are performed with the patient in a prone position. Consequently, the cement cannot be injected into the bone cavity when the viscosity is low enough to prevent interfacial pore formation. The cement will flow out of the cavity into the wound, causing contamination and possible necrosis. Conversely, it is also critical that the cement be sufficiently viscous so that there is little movement of the stem component of the prosthesis after placement before the cement is fully cured. For this reason, surgeons routinely wait for about three quarters of the cure time before performing the insertion of the stem.
The desirability for low viscosity of the bone cement to minimize interfacial porosity and the likelihood of failure, competes with the need for sufficiently high viscosity to prevent movement of the stem component of the prosthesis after placement. A continuing need exists for improvements in systems and methods for implanting prosthesis to reduce failure rates in orthopedic implant procedures.